The relevance of increased methacholine airway responsiveness detected in children with no current or past symptoms of asthma is not known. We wished to determine whether the presence of airway hyperresponsiveness in asymptomatic children is also associated with abnormal variability of peak expiratory flow (PEF). In 12 asymptomatic children with methacholine hyperresponsiveness, we examined the diurnal variation of peak expiratory flow (PEF) and response to inhaled bronchodilator. Twelve asthmatic children with a comparable range of methacholine hyperresponsiveness, and 12 normal children without methacholine responsiveness, were used as positive and negative controls. The children were aged 11 (range 9-14) yrs. The mean diurnal variation of PEF in those children with asymptomatic hyperresponsiveness was increased at 9.3%, to a degree comparable to the symptomatic asthmatic children (10.7%), and greater than the normal children (5.7%). Methacholine stimulated airway constriction was associated with symptoms in subjects from each group, indicating that the children were capable of perceiving airway constriction. We conclude that asymptomatic children with methacholine airway hyperresponsiveness have other evidence of mild variable airflow obstruction with increased diurnal PEF variability, and can perceive airflow limitation. The lack of symptoms in the children with airway hyperresponsiveness could be due to an insufficient stimulus to cause symptomatic obstruction, or the absence of eosinophilic airway inflammation, which may be a requirement for the development of symptomatic airway hyperresponsiveness.